Reforming is practiced widely throughout the world and is one of the most employed hydrocarbon processing reactions. In reforming, naphthene rings derived from paraffins are dehydrogenated into aromatic rings in the presence of a catalyst. The reformate will usually contain from 35 to 60 percent by weight of benzene, toluene and xylenes. Reforming catalysts are usually noble metals, such as platinum, or mixtures of platinum metals such as platinum and rhenium, on acidic supports such as alumina. Potential problems common to reforming processes include polynuclear aromatic (hereinafter may be abbreviated “PNAs”) content in the reformate and heat balance in the overall endothermic catalytic process.
If PNAs are not already present in the feed, they may be formed in the reforming processes. PNAs can form coke on the catalyst and foul units. Typically, PNAs include compounds having a plurality of fused aromatic rings and include compounds such as coronene and ovalene. As a result, it is desirable to remove PNAs from the one or more streams containing reformate to minimize catalyst deactivation through coking Adsorbent beds may be utilized to remove polynuclear aromatics from such reformate streams. After the adsorption capacity of the adsorbent is exhausted, the adsorbent may be disposed or regenerated.
U.S. Pat. No. 4,804,457 teaches the use of inter reactor PNA adsorption traps situated in a reforming process intermediate endothermic reforming reactors to remove any PNAs formed in the reforming process. The adsorption zone has an inorganic oxide selective for the separation of PNAs from mononuclear aromatics and normal paraffinic saturated hydrocarbons. The reference teaches that the separation to remove the PNAs from other hydrocarbons by adsorption is performed at a low temperature including from about 50° F. to 600° F.
U.S. Pat. No. 5,583,277 teaches that M41S, a molecular sieve, may be used to remove trace amounts of PNAs from reformate. U.S. Pat. No. 4,608,153 teaches the removal of PNAs using an iron-catalyst at high temperatures to selectively hydrogenate and hydrocrack the PNAs. GB1400545A teaches the removal of PNAs from gasoline or catalytic reformate using a graphite and alumina binder.
However, none of the references have provided a highly economical and efficient process for removing PNAs from one or more reformate streams. The process described herein calls for using activated carbon adsorbents in an adsorption zone located between at least two reforming reactors in a series of reactors, or in an adsorption zone located at the effluent of the last of a series of reforming reactors. The adsorption zone is able to operate at temperatures similar to those used in the reforming reactors, thus saving utilities by eliminating cooling and reheating steps required in previous processes.